The present invention relates to viaduct systems for rail and rapid transit lines.
The current technology used for rail and rapid transit viaducts is based on the experience developed for road viaducts. FIG. 1 illustrates a prior art viaduct structure wherein sets of rails are supported on a concrete platform or deck. The deck is mounted on top of a plurality of longitudinal support beams which are in turn mounted on top of transverse pier caps cast from concrete. The longitudinal support beams are steel girders. In another prior art construction, illustrated in FIG. 2, the rails are supported on top of a longitudinal box section mounted on piers. The box section is cast from concrete.
The prior art viaduct structures are disadvantageous from the standpoint of cost, aesthetics, safety and noise. In most rapid transit designs, a minimum clearance height is required between the ground and the rail support structure, with a 15 foot minimum being typical. In prior art structures, the actual height of the rapid transit vehicle is substantially above the minimum clearance height. In the construction of FIG. 1, the rail height is determined by the combined depth of the pier cap, the longitudinal support girders and the concrete deck. In the construction of FIG. 2, the rails are placed on top of the full depth of the load bearing box section. In either construction, it is impractical to minimize track height by reducing the depth of the longitudinal load bearing structures. Indeed, a design constraint of railway viaduct systems is that vertical load deflections be kept to a minimum to reduce the possibility of derailment. The longitudinal load bearing structures should thus have good bending stiffness, which is achieved most efficiently with tall bending sections having large moments of inertia.
FIGS. 1 and 2 illustrate the relative disadvantages of the prior art designs in terms of added rail height, as represented by the difference between the track level and the street clearance. As a result of this excessive rail height, increased costs are incurred for additional pier foundation materials in order to withstand transverse loads induced by trains on the substructure. Such loading creates bending moments at the pier foundations in direct proportion to track height. Additional costs are also incurred as a result of having to build higher station platforms in order to reach the track level.
From an aesthetics standpoint, the increased height of the prior art viaduct structures means that the entire structure is more visible from ground locations. From a safety standpoint, the prior art designs do nothing to reduce the possibility of collisions should a derailment occur. As to noise, there are often no structures provided to minimize vehicle sound levels at ground level.
A further disadvantage of prior art viaduct structures made from precast segmental sections and built span-by-span, whether for rapid transit or other purposes, stems from the manner in which such structures are post-tensioned. Conventional technology in precast segmental viaduct structures built span-by-span is to assemble one span over a steel truss, and to place and stress a plurality of post-tensioning cables that follow the parabolic diagram of load moments. The post tensioning cables are generally anchored at both ends of each span. As shown in FIG. 3, provision must be made in the viaduct segments for routing the cables along the moment curve, whatever the path of the moment curve. Each segment must therefore be uniquely fabricated to support the cables in their proper position.